1. Introduction Detonation Spraying Is a Thermal Spray Process Whereby

ARCHIVESOFMETALLURGYANDMATERIALS Volume 59 2014 Issue 4 DOI: 10.2478/amm-2014-0226

J. BOROWIECKA-JAMROZEK∗

DETONATION-SPRAYED WC-12%Co AND WC-17%Co DIAMOND-IMPREGNATED SEGMENTS

NATRYSKIWANIE DETONACYJNE POWŁOK WC-12%Co i WC-17%Co NA SPIEKI METALICZNO-DIAMENTOWE

This paper presents results of investigations of WC-12%Co and WC-17%Co coatings detonation sprayed on a sintered cobalt substrate. The main objective of the present work was to establish a new production route of sandwich type diamond-impregnated segments for circular sawing of stone and other construction materials. Unalloyed cobalt was chosen as the substrate material. Prior to coating the specimens were made by means of hot pressing of Extraﬁne cobalt powder in a graphite mould. The segments were then coated with WC-Co. The eﬀects of the coating process on the thickness, microhardness, microstructure and wear resistance of the deposits were investigated. The properties of the coatings were established by performing the following tests: a microstructure analysis, a point analysis of chemical composition and a linear analysis, a phase composition, a microhardness tests and abrasion resistance. Keywords: cobalt, sinter, hot pressing, diamond impregnated tools

W pracy przedstawiono wyniki badań detonacyjnego nanoszenia powłok WC-12%Co WC-17%Co na spieki kobaltu. Badania prowadzono w celu opracowania nowej technologii wytwarzania trójwarstwowych segmentów metaliczno-diamentowych typu sandwich przeznaczonych do produkcji tarczowych pił diamentowych i innych materiałów budowlanych. Próbki do badań otrzymano z proszków kobaltu gatunku Extraﬁne techniką prasowania na gorąco w graﬁtowej matrycy, a następnie na ich powierzchnię natryskano powłoki WC-Co. Badano wpływ parametrów procesu natryskiwania na grubość, mikrotwardość, mikrostrukturę i odporność na zużycie ścierne uzyskanych powłok. Przeprowadzono następujące badania uzyskanych powłok: obserwację mikrostruktury, analizę punktową i liniową składu chemicznego, analizę składu fazowego, pomiar mikrotwardości oraz badanie odporności powłok na zużycie ścierne.

1. Introduction haviour [5,6]. Although uniform and simple-shaped segments are cheaper to manufacture, the application requirements often Detonation spraying is a thermal spray process whereby justify selection of complex designs. A three-layer (sandwich high density, hard and high adhesion coatings can be deposit- type) structure of segments has been found the best choice for ed on a substrate material in order to increase its resistance most of the circular sawing applications. As the outer layers to wear [1-2]. The process consists in using repeated gas ex- differ from the inner one in their susceptibility to wear a de- plosions to heat and accelerate particles of a suitable powder sirable saddle-like wear profile is being developed at work on material through a detonation gun barrel and to throw them the working face of each segment (see Fig. 1) which imparts at a high velocity onto the substrate located adjacent to the a self-guiding characteristic to the saw blade and prevents it outlet of the barrel [3]. By careful pre-treatment of the sub- from deviating in the cut. strate surface and control of the spraying regimes it is possible The sandwich structure of the segment is produced by to deposit metallic coatings of a predetermined thickness and cold pressing the three layers of suitably prepared diamond microstructure [4]. grit-matrix powder mixtures. The green segments are subse- The main objective of the present work is to implement quently hot pressed to near-full density in graphite moulds the detonation spraying technology to reinforce the sides of [7,8,9]. uniform diamond-impregnated segments located on the cir- As the production of thin sandwich segments for small cumference of circular saw blades used for cutting natural saw blades is arduous and expensive the application of det- stone, concrete, brickwork and other ceramic materials. onation spraying to reinforce the sides of uniform segments The diamond-impregnated segments are exclusively pro- by deposition of wear resistant WC-Co coatings has been produced by powder metallurgy in a variety of shapes and internal posed as an alternative technology. structures that may greatly affect the tool life and its cutting be-

∗ KIELCE UNIVERSITY OF TECHNOLOGY, FACULTY OF MECHATRONICS AND MACHINERY DESIGN, AL. TYSIĄCLECIA PP 7, 25-314 KIELCE, POLAND 1328

Portions of WC-12wt% Co and WC-17wt%Co powder, having a narrow range of particle sizes between 22-45 µm, were repeatedly ejected at 4Hz for 6 minutes from a 580 mm barrel at a velocity of around 1000 m/s to be deposited on the cobalt target positioned 300 mm head of the barrel outlet. The properties of the coatings were established by performing the following tests: a microstructure analysis, a point analysis and a linear elements distribution analysis using a Je- ol JSM 5400 scanning microscope equipped with an ISIS 300 energy dispersive X-ray spectrometer (EDS), a phase composition analysis using a Bruker D-8 Advance X-ray diﬀractome- ter, a microhardness test, and abrasion resistance test using a T-07 tester.

3. Results Fig. 1. Structure of a sandwich-type segment and saddle-shaped wear of the working surface The analysis results showed that the sinters had a density of 8.73 ±0.01 g/cm3, which was close to the theoretical density, a high hardness of 271 ±4 HV10 ( the scatter bands 2. Experiment were estimated at 95% confidence level) and a fine-grained microstructure, as shown in Fig. 3. It has been concluded that A commercial Extrafine cobalt powder (Umicore, Bel- further research is required. gium), having Fisher sub-sieve size of around 1.5 µm, was used to manufacture the target material for the detonation spraying experiments. The powder is shown in Fig. 2. The substrate specimens were prepared by means of hot pressing the cobalt powder in a graphite mould at 850◦C for 2 minutes under a pressure of 35 MPa. All sintered specimens were tested for density and hardness. The density was determined by weighing them first in air and then in water. Their hardness was established at a load of 10kG using the Vicker’s hardness test method. Then the specimens were analyzed using a Leica DM4000 light microscope in order to determine their microstructure, respectively.

Fig. 3. Microstructure of the sinters Co (EF), the Nomarski contrast

The microstructure of the coatings obtained by detonation spraying are shown in Fig. 4.

Fig. 2. Extraﬁne cobalt powder Fig. 4. Microstructure of the coatings: (a) WC-12%Co, (b) WC-17%Co The coatings were produced using a Perun-S valveless dispenser at the Surface Advanced Technology Inc. in War- saw. To used the follows powders: AMI 5109.1 (WC-12%Co) The multi-layer microstructure of the coatings results of the company Amil Werkstoﬀtechnologie GmbH and Dia- from the strong deformation of the powder particles, which malloy 04152 (WC-17%Co) of the company Sulzer Metco. occurs during the deposition process. The layers vary consid- Prior to coating, the surfaces were degreased with acetone erably in the oxygen content, as shown by the point and linear and grit-blasted with electrocorundum EB 14. chemical content analyses (Fig. 5, 6, 7). 1329

Fig. 5. EDS analysis of the coatings: (a) WC-12%Co, (b) WC-17%Co

Fig. 7. Point analysis by EDS of the WC-17%Co coating: dark-coloured component (1), light-coloured component (2)

Phase identification using the X-ray diffraction technique revealed considerable differences in the composition of the carbide phase in the coatings. The WC-12%Co coating con- tained WC and a small amount of W2C (Fig. 8).

Fig. 8. Diﬀractograms of the coatings (a) WC-12%Co and Fig. 6. Point analysis by EDS of the WC-12%Co coating: the (b) WC-17%Co dark-coloured component (1), the light-coloured component (2) 1330

The content of WC and W2C carbides in the WC-17%Co rate of wear of the WC-17%Co coating, which is twice as coating increased significantly. The analysis revealed also the high as that of the WC-12%Co coating, is attributable to a presence of another carbide - (W,Co)6C. decrease in the content of WC at the expense of W2C and A change in the composition of the carbide phase was (W,Co)6C. probably due to the partial combustion of carbon during the detonation spraying of the coating. The porous structure of the Diamalloy 04152 powder particles was also of significance; it 4. Conclusion allowed diffusion of the oxidizing gases. The formation of complex M6C-type carbides is regarded The experimental results confirm that the gas detonation to be an unfavourable phenomenon. A decrease in the content method is well-suited for depositing WC-Co coatings with of the binding cobalt phase may lead to a considerable de- high hardness and different chemical and phase composition. crease in the strength and plastic properties of the coating The coatings produced from WC-12%Co powder possessed material. higher hardness and nearly twice as high abrasion resistance The hardness of the WC-12%Co coating was much higher as the WC-17%Co coatings. Both coatings were well adhered than that of the WC-17%Co coating (Fig. 9). After a Vicker’s to the cobalt substrate. As a result, the strength of the sub- hardness test, there were no cracks around the indentation cor- strate surface layer, several dozen micrometers in thickness, ners, which indicates that the coatings have appropriate plastic was improved significantly by cold working. properties. No cracking or delamination around the indenta- During coating deposition, the temperature of the sub- tions at the interface between the coating and the substrate strate material did not exceed 150◦C; it had virtually no ef- confirms good adherence of the coating to the substrate. The fect on the properties of the diamond crystals. It can be con- strength of the substrate surface layer was improved by cold cluded that the method is suitable for increasing the abra- working, which involves collisions of the WC-Co powder par- sion resistance of the segment side surfaces. Thin WC-Co ticles with the sinter surface. coatings applied to the side surfaces of the homogeneous metallic-diamond segments ensure uniform lateral wear of the saw. This can be an alternative to sandwich-type segments. The analysis described in this paper was qualitative in nature. No tests were conducted to estimate the efficiency of the tool. Reliable tests to assess the durability of metallic-diamond tools are static in nature, time-consuming and very costly. This paper deals with qualitative analysis only; the wear rate was typical of saws used for cutting sandstone slabs. Fig. 9. Relationship between the microhardness and the distance from the surface REFERENCES

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Received: 20 January 2014.